Tar sand, also known as oil sand and bituminous sand, is now well recognized as a valuable source of hydrocarbons. There are presently two large plants producing synthetic crude oil from the tar sands of the Athabasca region of Alberta. In these operations, the tar sands are first mined and the bitumen is then extracted from the ore by a process called the hot water process. The recovered bitumen is subsequently upgraded in a hydrotreating facility to produce the synthetic crude.
The physical nature of the Athabasca tar sand itself is what makes it amenable to the hot water process. More particularly, the tar sand is composed of bitumen, water, quartz sand and clays. The minute clay particles are contained in the water. The water forms a film around each sand grain. And the bitumen or oil is disposed in the interstices between the water-sheathed grains. Because the bitumen is in the water phase, it can be displaced from the sand grains by a water addition mechanism.
The first two steps of the hot water process, referred to as `conditioning` and `flooding`, therefore are designed to aerate the slurry and disperse or increase the separation of the oil flecks away from the sand grains. A subsequent flotation/settling step is then applied to recover the oil and sand as separate products.
A "process aid" (commonly NaOH) is usually provided as an additive in the conditioning step. This process aid appears to react with groups associated with the bitumen molecules to form surfactants. In addition, there are naturally occurring surfactants present in discrete form in the tar sand. These various surfactants play an important role in facilitating successful dispersion and flotation of the oil.
The present invention is concerned with managing the process to ensure a favorable surfactant regime in the slurry.
The `hot water process` will now be described in a general fashion. It is also disclosed in greater detail in the prior art literature and patents.
In the first step, `conditioning`, the as-mined tar sand is mixed with hot water (180.degree. F.) and NaOH in a rotating horizontal drum. Steam is sparged into the drum contents at intervals along its length to ensure a slurry exit temperature of about 180.degree. F. Typically, the amounts of reagents added are in the following proportions:
______________________________________ tar sand 3250 tons hot water 610 tons NaOH 4 tons (20% NaOH) ______________________________________
The residence time in the drum is typically 4 minutes.
As previously stated, during conditioning the slurry is aerated in the course of being agitated and the solids and bitumen are dispersed in the aqueous phase.
The slurry leaving the drum is screened, to remove oversize material. The screened slurry is then `flooded` by diluting it with a large dose of hot water. The flooded product typically comprises:
______________________________________ bitumen 7% by weight water 43% solids 50% ______________________________________
The product temperature is typically 160.degree.-180.degree. F.
The diluted slurry then is transferred into a thickener-like flotation vessel, referred to as a `primary separation vessel` ("PSV"). This open-topped vessel has a cylindrical upper end and a conical lower end.
The slurry is retained for a period of time in the PSV under quiescent conditions. Typically the retention time is about 45 minutes.
In the PSV, most of the sand sinks and is concentrated by the conical bottom to form a sand layer. This sand is discharged through a bottom outlet as an underflow. The discharge is discarded and is referred to as `primary tailings`.
Much of the bitumen becomes attached to air bubbles and rises to form a layer of froth on the surface of the aqueous phase. This froth, referred to as "primary froth", overflows into a launder and is separately recovered.
______________________________________ bitumen 66.4% by weight solids 8.9% water 24.7% ______________________________________
Not all of the bitumen is sufficiently buoyant to rise into the primary froth layer. Much of this non-buoyant bitumen, together with a large part of the clays, forms an aqueous suspension between the sand and froth layers. This suspension is referred to as "middlings". The water phase of the suspension can be referred to as "process water".
A stream of middlings is withdrawn from the vessel and is fed into sub-aerated flotation cells. In these cells, the middlings are subjected to vigorous agitation and aeration. Bitumen froth, termed "secondary froth", is produced and recovered. This secondary froth typically comprises:
______________________________________ bitumen 23.8% by weight solids 17.5% water 58.7% ______________________________________
It will be noted that the secondary froth is considerably more contaminated with water and solids than the primary froth.
Before being forwarded on to the upgrading operation, it is necessary to remove most of the solids and water from the bitumen. This cleaning procedure is carried out in two stages of centrifugation. However, the secondary froth is not as easy to clean as the primary froth.
For this and other reasons, it is highly desirable in the management of the hot water process to maximize the production of primary froth and to minimize the production of secondary froth.
It is well understood in the industry that the tar sand feed varies significantly in nature. These changes in tar sand nature have a dramatic impact on the proportion of the contained bitumen that is recovered and whether recovered bitumen reports as primary froth or secondary froth. Factors which affect the nature of the tar sand include:
the relative proportions of bitumen, water, and "fines" (i.e. solids which pass through a 325 mesh screen) in the feed; PA0 the extent of "weathering" or aging of the ore, which occurs after it is mined but before it is processed; and the circumstances under which the particular species of tar sand was laid down. PA0 that there was a connection between free surfactant concentration in the process water and primary froth recovery; PA0 more particularly, it was taught that if one monitored the "free" surfactant concentration in the process water when a single tar sand feed was processed at different levels of NaOH addition (all other conditions being constant), and if one plotted carboxylate-type free surfactant concentrations against primary froth recovery, a peak-like curve (referred to as a "processibility curve") was developed; and PA0 that if one repeated this procedure in the same circuit using different tar sand feeds, the various processibility curves developed all yielded their peak at substantially the same free surfactant concentration. PA0 a first class of surfactants that appear to originate from carboxylate groups; and PA0 a second class, more polar in nature, that appear to originate from sulfonate groups; PA0 determining a measure of the critical equilibrium free surfactant concentration value for the circuit for the carboxylate-type surfactants (which value is hereafter referred to as "C.sub.cs.sup.o "); PA0 determining a measure of the critical equilibrium free surfactant concentration value for the circuit for the sulfonate-type surfactants (which value is hereafter referred to as "C.sub.ss.sup.o "); PA0 determining for the ore currently being treated whether the carboxylate-type or the sulfonate-type surfactants first predominantly influence the maximum primary froth recovery at low process aid addition; PA0 and then adjusting process aid addition to the hot water process so as to bring the concentration of the dominating class of surfactants toward the critical concentration thereof.
Some tar sands are referred to as "rich"--they typically contain 12-14% (w/w) bitumen and a relatively low fines content. Others are referred to as "lean"--they typically contain 6-9% bitumen and a relatively high fines content. Sample compositions are given in Table I.
TABLE I ______________________________________ Bitumen Water Solids Fines Oil Sand (% w/w) (% w/w) (% w/w) (% w/w) ______________________________________ rich 14 1 85 14 average 11 3 86 19 lean 6 11 83 21 ______________________________________
Generally stated, rich tar sands process easily, giving a high recovery of relatively clean bitumen. Lean tar sands process poorly, giving a low recovery of relatively dirty bitumen.
In summary then, it is always a prime objective of a hot water process operator to manage the process so as to maximize recovery and to ensure that the greatest possible proportion of the bitumen recovered is in the form of primary froth. But his efforts in this direction are often interfered with by the variations in the nature of the tar sand feed.
In our U.S. Pat. No. 4,462,892 and in our paper entitled "The influence of natural surfactant concentration on the hot water process for recovering bitumen from the Athabasca oil sands", AOSTRA J. Research, 1 (1984) 5, (incorporated herewith by reference), we disclosed a process for better managing the hot water process. In these references, it was disclosed:
Stated otherwise, primary froth oil recoveries were observed to pass through a distinct maximum as a function of the equilibrium free carboxylate-type surfactant concentration in the process water. And the maximum oil recoveries were associated with a single valued critical equilibrium free surfactant concentration, which critical value would hold for a wide variety of types of oil sand when treated in that particular circuit.
(By "free" surfactant is meant those surfactant moities in solution and not bound up at interfaces. By "extraction circuit" is meant the conditioning drum, PSV and connecting piping.)
Thus, for a given circuit, an operator can establish the critical equilibrium free surfactant concentration ("C.sub.cs.sup.o ") by making several runs with a single feed at varying NaOH additions; he can then monitor the equilibrium free surfactant concentration ("C.sub.cs ") in the process water for various tar sands fed to the process; and he can adjust the NaOH addition (as well as other process parameters such as water addition) to bring C.sub.cs to C.sub.cs.sup.o and thereby maximize primary froth production.
The equilibrium free surfactant concentration in a sample of process water can be established by a method described in our paper entitled "A surface-tension method for the determination of anionic surfactants in hot water processing of Athabasca oil sands", published in Colloids and Surfaces, 11 (1984), 247-263. This paper is incorporated herewith by reference.
The mining of tar sands involves excavating a trench nearly 5 km in length and hundreds of feet in depth. The excavating equipment moves along the face of the trench and gradually increases the width of the trench. In the course of making a pass along the trench, many quite different varieties of tar sand are mined. For the majority of these ores, the process set forth in U.S. Pat. No. 4,462,892 is satisfactory. More particularly, with these ores the quantity of NaOH addition can be adjusted within a reasonably narrow range to bring C.sub.cs equal to C.sub.cs.sup.o and maximum primary froth production will be attained.
However, it has been found that there are certain pockets of tar sand ore that do not initially appear to be most advantageously processed by practicing the process of U.S. Pat. No. 4,462,892. These ores, referred to as `anomalous ores`, have been found to give very poor primary froth recoveries when processed in accordance with U.S. Pat. No. 4,462,892.
An examination of the compositions of these anomalous ores did not give any useful guidance as to what might be done to improve bitumen recovery from them. When NaOH addition was varied within the commonly used range for the circuit, little or no improvement was noted.
There was therefore a need for an understanding of what was affecting the process and causing the poor recoveries with respect to these anomalous ores--and there was a further need for a means for overcoming the difficulty and modifying the extraction process to make it work well when treating them.